A receiver, also known as User Equipment (UE), mobile station, wireless terminal and/or mobile terminal is enabled to communicate wirelessly in a wireless communication system, sometimes also referred to as a cellular radio system. The communication may be made e.g. between two receivers, between a receiver and a wire connected telephone and/or between a receiver and a server via a Radio Access Network (RAN) and possibly one or more core networks.
The receiver may further be referred to as mobile telephones, cellular telephones, computer tablets or laptops with wireless capability. The UEs in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another receiver or a server.
The wireless communication system covers a geographical area which is divided into cell areas, with each cell area being served by a radio network node, or base station e.g. a Radio Base Station (RBS), which in some networks may be referred to as transmitter, “eNB”, “eNodeB”, “NodeB” or “B node”, depending on the technology and terminology used. The radio network nodes may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the radio network node/base station at a base station site. One radio network node, situated on the base station site, may serve one or several cells. The radio network nodes communicate over the air interface operating on radio frequencies with the receivers within range of the respective radio network node.
In some radio access networks, several radio network nodes may be connected, e.g. by landlines or microwave, to a Radio Network Controller (RNC) e.g. in Universal Mobile Telecommunications System (UMTS). The RNC, also sometimes termed Base Station Controller (BSC) e.g. in GSM, may supervise and coordinate various activities of the plural radio network nodes connected thereto. GSM is an abbreviation for Global System for Mobile Communications (originally: Groupe Special Mobile).
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), radio network nodes, which may be referred to as eNodeBs or eNBs, may be connected to a gateway e.g. a radio access gateway, to one or more core networks.
In the present context, the expressions downlink, downstream link or forward link may be used for the transmission path from the radio network node to the receiver. The expression uplink, upstream link or reverse link may be used for the transmission path in the opposite direction i.e. from the receiver to the radio network node.
In order to divide forward and reverse communication channels on the same physical communications medium, when communicating in the wireless communication system, a duplexing method may be applied such as e.g. Frequency-Division Duplexing (FDD) and/or Time-Division Duplexing (TDD). The FDD approach is used over well separated frequency bands in order to avoid interference between uplink and downlink transmissions. In TDD, uplink and downlink traffic are transmitted in the same frequency band, but in different time intervals. The uplink and downlink traffic is thus transmitted separated from each other, in the time dimension in a TDD transmission, possibly with a Guard Period (GP) in between uplink and downlink transmissions. In order to avoid interference between uplink and downlink, for radio network nodes and/or receivers in the same area, uplink and downlink transmissions between radio network nodes and receivers in different cells may be aligned by means of synchronization to a common time reference and use of the same allocation of resources to uplink and downlink.
In order to enable coherent demodulation of data, the radio network node (e.g., eNodeB) has to send a pre-defined reference signal, aka pilot signal, to the receiver (e.g., UE). The reference signal may not encode any information and it is typically known to the receiver, e.g. by being predefined. Using a priori information on the modulation symbols and time-frequency location of the transmitted reference signal, the receiver may, based on the received reference signal, obtain channel estimates (e.g., the phase and amplitude of the channel frequency response), which may be used for channel equalization prior to the demodulation.
In the prior art 3GPP LTE system, multiple transmit and receive antennas are supported and the notion of antenna port is used. Each downlink antenna port is associated with a unique reference signal. An antenna port may not necessarily correspond to a physical antenna and one antenna port may be associated with more than one physical antenna. In any case, the reference signal may be used for channel estimation for data that is transmitted on the same antenna port. Channel estimation therefore may be performed for all antenna ports that are used for the data transmission.
In LTE, the smallest time-frequency entity that can be used for transmission is referred to as a Resource Element (RE), which may convey a complex-valued modulation symbol on a subcarrier. A Resource Block (RB) comprises a set of REs and is of 0.5 ms duration (e.g., 7 Orthogonal Frequency-Division Multiplexing (OFDM) symbols) and 180 kHz bandwidth (e.g., 12 subcarriers with 15 kHz spacing). The transmission bandwidth of the system is divided into a set of RBs. Each transmission of user data is performed over 1 ms duration (which is also referred to as a subframe) on one or several RBs.
OFDM is a method of encoding digital data on multiple carrier frequencies. OFDM is a Frequency-Division Multiplexing (FDM) scheme used as a digital multi-carrier modulation method. A large number of closely spaced orthogonal sub-carrier signals are used to carry data. The data is divided into several parallel data streams or channels, one for each sub-carrier.
OFDM has developed into a popular scheme for wideband digital communication, whether wireless or over copper wires, used in applications such as digital television and audio broadcasting, Digital Subscriber Line (DSL; originally: Digital Subscriber Loop) broadband internet access, wireless networks, and 4G mobile communications.
A number of reference signals have been defined in the LTE downlink, such as e.g.:
Common Reference Signal (CRS)
CRS is a cell-specific reference signal, which is transmitted in all subframes and in all RBs of the carrier. The CRS serves as a reference signal for several purposes, comprising inter alia: demodulation; Channel state information measurements; Time- and frequency synchronization, and/or Radio Resource Management (RRM) and mobility measurements.
Up to 4 CRS antenna ports may be accommodated. Since the CRS provides for multiple purposes, it has to be rather robust and hence has quite a large density. The disadvantage is that its overhead is significant, which deteriorates the general efficiency of the wireless communication system, as resources that otherwise may be utilized for transmitting data are dedicated to reference signalling of CRS.
With multiple antennas, it is possible to achieve beamforming by applying different complex-valued precoder weights on the different antennas. However, since the CRS is cell-specific, it cannot be UE-specifically precoded, i.e., it cannot achieve any beamforming gains. On the other hand, the user data on a data channel such as e.g. the Physical Downlink Shared Channel (PDSCH) may undergo beamforming since it is not cell-specific.
A different approach for reference signal design was subsequently introduced in the LTE system, comprising two new reference signals, each having a specific purpose.
Channel State Information Reference Signal (CSI-RS)
CSI-RS is a sparse UE-specific reference signal used only for estimating channel state information by means of a transmission quality indicator such as e.g., Channel Quality Indicator (CQI), Precoding Matrix Indicator (PMI) and/or Rank Indication (RI), which the receiver reports to the radio network node (eNodeB). The CSI-RS may be transmitted in all RBs of the carrier but with a configurable period in time and it is much sparser than the CRS. Up to 8 CSI-RS antenna ports may be accommodated.
Demodulation Reference Signal (DM-RS)
DM-RS is a receiver-specific reference signal used only for phase and amplitude reference for coherent demodulation, i.e., to be used in channel estimation. In contrast to the CRS, it is only transmitted in the RBs and subframes where the receiver has been scheduled data (i.e., containing the PDSCH). Up to 8 DM-RS antenna ports may be accommodated. The antenna ports (labelled 7-14) are multiplexed both in frequency and by orthogonal cover codes. Since it is receiver-specific, the DM-RS may be precoded with the same precoder used for the PDSCH, hence beamforming gains could be achieved for the reference signal. When using the same precoder for the data symbols as for the reference signal symbols, the precoding becomes transparent to the receiver which may observe the precoder as part of the channel. Hence, the precoder is not signalled to the receiver. Typically different precoders may be used in different RBs, implying that channel estimates cannot be interpolated between RBs. However, the system may configure the UE such that the same precoder is used on a set of contiguous RBs (i.e., RB bundling). This allows interpolation in frequency domain between RBs in order to enhance channel estimation performance. In the time-domain, no such precoder restrictions apply and channel estimation interpolation between subframes cannot be performed.
Additionally, the DM-RS is also utilized for demodulation reference for some of the downlink control channels, e.g., the enhanced Physical Downlink Control Channel (ePDCCH) which does not apply CRS-based demodulation. There is currently no standardized transmit diversity scheme for the ePDCCH, which is in contrast to the PDCCH. Under severe channel conditions, the performance of the ePDCCH may therefore be worse than the CRS-based PDCCH. In the absence of a CRS, it is therefore crucial that the DM-RS supports reliable operation of the ePDCCH.
In the legacy LTE system, at least one CRS port is always transmitted, regardless of the number of configured CSI-RS antenna ports and DM-RS antenna ports. Thus even if DM-RS is used as a reference signal for demodulation, the CRS is transmitted.
Furthermore, the LTE system is capable of carrier aggregation, wherein the receiver may simultaneously receive on multiple downlink component carriers (or serving cells, as they also may be referred to as). One of the serving cells/carriers is referred to as the Primary Cell (PCell) and the other cells/carriers are Secondary Cells (SCells). It has been proposed to define cells (e.g., SCells) which do not transmit any CRS antenna port. Therefore, for such cells, data channel and downlink control channel transmissions would have to rely upon DM-RS based demodulation.